Job label should be unique

Tried creating the unique constraint on the model but it creates problems with the job serializer. Could be that need to implement a uniquevalidator in the serializer. Removed the unique constraint from the model for now.

Locate the job model and job serializer, then trace how duplicate labels are currently handled. Compare model uniqueness with serializer validation; done means duplicate job labels are rejected without breaking job serialization.

Written by the indexing model from the issue text.